1. Technical Field
This disclosure relates generally to a color printing apparatus, such as an ink jet printing apparatus, a color copier, etc., and in particular to a color printing apparatus that superposes pattern data, which, for example, can be used for identifying a counterfeiting instrument, on image data in a memory.
2. Discussion of the Related Art
Recent advances in image resolution, image processing, and printing enable high precision color image printing at relatively lower cost. For example, a color printing apparatus employing an ink jet process can be used in a printing process which is capable of outputting a high precision and gradation color image at low cost. As a result, paper money and variable security and so on unfortunately can be counterfeited by employing such high precision input instruments.
In order to prevent such counterfeiting, a function of detecting specified information related to the paper money and either triggering an alarm or disenabling an input or output operation is generally included in input and printing instruments. In addition to stopping the above-mentioned operation, a capability of identifying an instrument, which has generated an output, is given importance in view of the ever present objective of reducing a crime rate. Thus, information identifying the output instrument is sometimes written on printing material by a manner of some kind. For example, a fine pattern not perceptible to the human eye can be embedded in an image, as discussed in Japanese Patent Application Laid Open No. 2000-313156. Further, as discussed in Japanese Patent Application Laid Open No. 2002-240387, information can be written with colorless special ink generating a light upon receiving an ultraviolet light as a reaction, or information itself is embedded and reversely converted after being mapped in a frequency region.
Embedding information hardly perceptible to the human eye in image data is perhaps the simplest method among the methods mentioned above. Specifically, image data is printed with pattern data capable of identifying a printing instrument with the pattern data being superposed thereon without giving influence thereto. The pattern data can be extracted to identify the output instrument based upon an output image.
When an image is formed by an ink jet process, a method most frequently used is to perform two dimensional image formation that discharges ink in accordance with input image information in synchronism with movement of a carriage in a main scanning direction while transporting recording medium in a sub scanning direction.
With such a recording process, printing efficiency (for example, a number of printing sheets per minute) can be improved by speeding up either sheet conveyance or movement of a carriage as fast as possible. For such a purpose, printing is sometimes shortened by either moving the carriage at higher speed in a data blank region than when printing or conveying a sheet at high speed in a blank in the sub scanning direction.
When such pattern data is to be superposed and output with image data, the above-mentioned operation cannot be speeded up and sometimes causes a problem, if a region of the image data is not detected. Since a blank entirely or substantially disappears from an original image if pattern data is excessively superposed even on the blank, an image is always output. As a result, the above-mentioned efficiency obtained by either moving the carriage or conveying a sheet at high speed is reduced.
It is desirable to solve these problems and to provide a color printing apparatus capable of superposing an instrument identifying pattern on an image while suppressing decrease in printing speed as little as possible.